The present invention relates to semiconductor device fabrication and, more particularly, to a method of fabrication quality control, based on monitoring the alignment of two layers deposited successively on a semiconductor wafer.
Semiconductor devices, such as processor chips and memory chips, are fabricated by the deposition of successive layers of substances such as polysilicon, silicon dioxide and various metals on a silicon wafer substrate. As shown in FIG. 1, the devices are fabricated as dies 10, separated by scribes 12, on a wafer 14. After each layer is deposited, it is covered with photoresist. The photoresist is exposed to a preselected pattern of light. Depending on the type of photoresist, a portion of the photoresist, either the portion that was exposed to the light or the portion that was not exposed to the light, is removed, usually by dissolution, exposing the layer beneath the photoresist. The thus exposed part of the new layer is either totally or partially removed, for example by etching, to provide the layer with its desired geometry.
The exposure of the photoresist to the light pattern is effected using a tool called a "stepper". The desired two-dimensional geometric pattern of the layer is embodied in a reticle, either in a transparent portion of the reticle or in an opaque portion of the reticle. Collimated light is directed through the reticle, and focused on a portion of the wafer. The portion of the wafer upon which the light is directed at any one time is known in the art as a "field". The fields in FIG. 1 are demarcated by dotted lines 16. In this example, each field spans four dies 10.
It is critical that successive layers be aligned accurately. For this purpose, the reticle includes alignment key portions in the portion of the reticle corresponding to scribes 12. FIG. 2 shows, schematically, a typical reticle pattern 18 , including die portions 20 corresponding to four dies 10, scribe portions 22 corresponding to scribes 12 that separated dies 10, and, in scribe portions 22, two alignment key portions 24 corresponding to Cartesian x and y axes. After the corresponding layer on wafer 14 has been provided with its desired geometry, as described above, the layer includes, in scribes 12, and as shown in FIG. 1, alignment keys 26 whose geometry matches the geometry of alignment key portions 24. The geometry of alignment key portions 24 is designed so that the two-dimensional pattern of alignment keys 26 shows up both in the layer in which alignment keys 26 are fabricated and in the immediately succeeding layer, so that wafer 14 can be positioned accurately by the stepper, relative to the reticle of the succeeding layer, for the accurate patterning of the succeeding layer relative to the layer that bears alignment keys 26. Because alignment keys 26 are used specifically by the stepper to effect this alignment, alignment keys 26 are termed "stepper keys" herein.
To verify the accuracy of the mutual alignment of two layers on wafer 14, reticle 18 also includes overlay key portions 28 that typically are square as shown. Overlay key portions 28 for successive layers have different sizes, with the overlay key portion of the lower of a pair of successive layers (termed herein the "first" layer) being larger than the overlay key portion of the upper of a pair of successive layers (termed herein the "second" layer). The two layers, when finally formed with their desired geometries, include corresponding square portions that are termed "overlay keys" herein. If the second layer is aligned accurately with the first layer, then the overlay keys of the second layer are at their nominal (designed) positions relative to the overlay keys of the first layer, i.e., exactly in the centers of the corresponding overlay keys of the first layer. Deviations of the actual positions of the overlay keys of the second layer, relative to the overlay keys of the first layer, from this central positioning are diagnostic of misalignment between the first and second layers. FIG. 3 shows a microphotograph of an overlay key 30 of a second layer of photoresist centered in an overlay key 34 of a first layer 36 of metal.
The stepper positions wafer 14, for exposure to light through the reticle corresponding to the second layer, by searching for and locating geometric patterns of stepper keys 26 that are located approximately in the expected ("nominal") positions of stepper keys 26. Usually, stepper keys 26 are not located precisely in their nominal positions, but instead are slightly displaced relative to their nominal positions. The stepper constructs a mathematical model of this displacement and positions wafer 14 accordingly to adjust the locations of fields 16 for the second layer. This model is termed herein the "stepper model".
After the second layer is fabricated, wafer 14 is transferred to an overlay measurement tool for evaluating the accuracy of the mutual alignment of the first and second layers, as represented by the accuracy of the mutual alignment of the first and second overlay keys. Like the stepper, the overlay measurement tool constructs its own mathematical model, termed herein the "overlay model", of the misalignment of the second layer relative to the first layer.
FIG. 4 shows six ways in which fields 16' of the second layer can be misaligned with respect to fields 16 of the first layer. FIG. 4A shows scaling: the overall size of the geometric pattern defined by fields 16' is larger than the overall size of the geometric pattern defined by fields 16. Isotropic scaling is shown in FIG. 4A. Scaling can also be anisotropic, with different magnifications in the x and y directions. FIG. 4B shows orthogonal x-rotation: fields 16' are skewed horizontally relative to fields 16. FIG. 4C shows rigid rotation of fields 16' relative to fields 16. FIG. 4D shows magnification of individual fields 16' relative to corresponding fields 16. FIG. 4E shows rotation of individual fields 16' relative to corresponding fields 16 as a consequence of reticle rotation. FIG. 4F shows vertical translation: fields 16' are shifted vertically relative to fields 16. For illustrational clarity, only nine fields 16 and 16' are shown in each of FIGS. 4A, 4B, 4C, 4D, 4E and 4F. The six modes of misalignment shown in FIG. 4, plus horizontal translation, are the degrees of freedom of the overlay model. (Orthogonal y-rotation can be expressed as a combination of rigid rotation and orthogonal x-rotation.) The same modes of misalignment, with the exception of magnification and reticle rotation, are the degrees of freedom of the stepper model.
The residuals of the overlay model, ie., the differences between the actual Cartesian (x,y) coordinates of the overlay keys of the first layer, in a coordinate system defined by the second layer, and the Cartesian coordinates of the overlay keys of the first layer that are predicted by the overlay model, are diagnostic of the quality of the overlay model. Large absolute values of the residuals indicate that the model fails to explain the misalignment. This failure is in turn diagnostic of problems in the fabrication process. Similarly, the residuals of the stepper model, ie., the differences between the measured Cartesian coordinates of the stepper keys and the coordinates of the stepper keys that are predicted by the stepper model, are diagnostic of the quality of the stepper model. In practice, only the overlay model residuals are used for quality control of the fabrication process, because only the overlay model is directly related to the actual degree of misalignment of the first and second layers. Although the stepper calculates stepper model residuals for each wafer 14 processed therein, these stepper model residuals are used only for calibrating the stepper during installation and for stepper maintenance.